In the field of medicine related to the effects of air pollution, it has been determined that mercury vapor can be harmful to the health of certain individuals if found in excessive concentrations in the atmosphere. It is considered practicable to utilize a method and apparatus for detection of mercury vapor pollution for the purpose of locating sources of mercury vapor pollution in order that such pollution may be controlled.
Three techniques are concurrently in use for determination of mercury vapor in atmosphere which are: (1) ultraviolet absorption (using the characteristic 2537 A mercury line) (various versions), (2) amalgamation followed by ultraviolet absorption, and (3) correlation spectrometry.
The detection limit of the simple resonant absorption methd is about 1 ng of mercury vapor in a 15 inch air path. This corresponds to roughly 1 ng/liter, which is 2 to 3 orders of magnitude higher than background concentrations. There is also even present the question of potential interference from other ultraviolet absorbers in the sample, such as water vapor, carbon dioxide, sulfur dioxide, hydrogen sulfide and various oxides of nitrogen. Although resonant absorption by mercury vapor is extremely selective, the concentrations of these other gases and vapors can be several orders of magnitude higher than that of the mercury vapor. Such interference has been found to present severe problems.
In an attempt to overcome these problems airborne survey work, where mercury vapor is detected by mercury detection equipment aboard aircraft, a refinement of the absorption technique has been utilized for accomplishing direct measurements in ambient air. Interference is minimized by utilizing the pressure broadening of the characteristic line emitted by a specific mercury vapor lamp. The cold vapor in the absorption path attenuates only the center portion of the line, whereas other ultraviolet absorbers attenuate the line entirely. The ratio of the signal from the center to that from the edges of the line is then obtained to reject interference. The resulting instrumentation is much more complex than a simple absorption cell, requiring precise temperature control of the lamp and special techniques to stabilize the photomultiplier detectors against drift relative to each other. A folded lightpath is utilized to increase sensitivity and still the detection limit is greater than 10 ng per cubic meter. Light losses especially at the mirrors also become a problem. Another version of this approach is to use a magnetic field to produce Zeeman splitting of the line, and to use this effect instead of pressure broadening.
An attempt has been made to adapt the principal of correlation spectrometry to direct mercury vapor determination. In this method the detected absorption spectrum (which usually contains many peaks) is mixed with an analog of the spectrum from the desired components. The output is proportional to the concentration of the desired components, free from spectral interferences. This method has not proved especially successful for direct mercury vapor determination.
It is believed that sample concentration is necessary to achieve the desired analytical performance for accurate detection of mercury vapor in air. This being so, it is logical to attempt to selectively concentrate the mercury and so simplify the analysis stage. This has been accomplished in the past by utilizing the amalgamation method of mercury detection. When the amalgmation method is employed, the mercury is collected by drawing the air to be analyzed past a gold or silver ribbon or mesh having a weight of about 1 g and a surface area of about 50 square centimeters. The mercury vapor is subsequently released into the analysis cell by radio frequency heating. The amalgamation is specific for mercury and the collection and release efficiencies are quoted to be in the order of 100 percent, thus insuring a quantitative analysis. At the detection limit, 1 ng of mercury is collected from up to one cubic meter of air. The resulting concentration in the analysis cell is about 1 ng/liter, yielding an effective concentration factor of 10.sup.3.
Utilizing this technique, adequate sensitivity for field analysis of soil gas samples is reported using very simple instrumentation. For airborne surveys, however, it is desirable to attain a detection limit of about 1 ng per cubic meter and if the analyses are to be performed in real time, the available sample collection time must be in the order of 0.1 to 5 minutes, based on a resolution of up to a few miles at a flying speed of 80 miles per hour. This implies that at least one cubic meter of air must be drawn past the amalgamator per minute necessitating a flow rate some two to three orders of magnitude higher than the maximum quoted for insuring one hundred percent amalgamation efficiency (1 cubic foot per minute).
It is desirable to provide a method of mercury vapor detection based on a rapid one hundred percent efficient collection of mercury vapor from the atmosphere by amalgamation onto a material that is provided with a thin coating of a noble metal and by subsequently removing collected mercury from the noble metal in such manner that it can be detected by means of x-ray fluorescent analysis. It is desirable to provide for collection, transfer and analysis of the mercury vapor in such manner that all possibility of erroneously high readings due to interference from other components present in vastly greater concentrations are effectively removed. It is also desirable that the method of collection and transfer be as near one hundred percent efficient as possible thereby insuring a quantitative rather than qualitative determination.
It is therefore a primary object of the present invention to provide a novel method and apparatus for detecting and measuring mercury vapor content present in the atmosphere.
It is an even further object of the present invention to provide a novel method and apparatus for efficiently detecting and measuring mercury content in the atmosphere, which method may be efficiently accomplished through use of simple and inexpensive collection and testing apparatus.
Among the several objects of the present invention is noted the provision of a novel method and apparatus for detection of the presence and amount of mercury vapor in an atmospheric environment which, although utilizing a known amalgamation approach, incorporates a feature of cascading in mercury vapor collection that insures one hundred percent amalgamation efficiency.
Other and further objects, advantages and features of the present invention will become apparent to one skilled in the art upon consideration of the written specification, the appended claims and the annexed drawings. The form of the invention, which will now be described in detail, illustrates the general principles of the invention, but it is to be understood that this detailed description is not to be taken as limiting the scope of the present invention.